The illicit use of chronic opioids continues to be a significant social problem. Although some of the chronic effects of opiates have been well characterized, the neuronal mechanisms of opioid tolerance and dependence remain elusive. These experiments will compare the acute and chronic effects of opiates on mu receptors and their second messengers, G-proteins, which will provide further insight into the opioid system and possibly into the biochemical mechanisms involved in opiate tolerance and dependence. Specifically, the objectives of this project are to test the following hypotheses: 1) chronic heroin treatment affects the ability of mu opioid receptors to couple to G-proteins, 2) Mu receptor coupled G-protein desensitization occurs at different rates across the rat brain and this desensitization may be related to the development of tolerance, and 3) chronic treatment with various opioid agonists produces desensitization of mu receptor coupled G-proteins that correlates with the intrinsic efficacy of agonist. Investigation of the neuronal mechanisms underlying opiate tolerance and dependence will provide insight into drug abuse and potentially provide the information necessary to create better treatment programs for opiate abuse.